1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor liquid crystal display, and more particularly to a method for forming a polycrystalline silicon thin film transistor by using sequential lateral solidification.
2. Description of the Prior Art
As generally known in the art, a thin film transistor (hereinafter, referred to as “TFT”) used as a switching element in a liquid crystal display, an organic light emitting display, etc., is an element having the most important role in performance of such flat panel display devices. Here, the mobility or current leakage, which serves as a standard for determining the performance of the TFT, largely depends on the state or structure of the semiconductor active layer, which is a passage through which charge carriers move, or the state or structure of the silicon thin film which is material of the semiconductor active layer.
Most TFTs of the current commercial liquid crystal display devices have an active layer made from amorphous silicon (hereinafter, referred to as “a-Si”). However, an a-Si TFT having an active layer made from amorphous silicon has a very low mobility of about 0.5 cm2/Vs, which makes it difficult to produce all switching elements in the liquid crystal display device. In other words, driving elements for peripheral circuits of the liquid crystal display device have to operate at a very high speed, and the a-Si TFT cannot satisfy such an operation speed as required by the driving elements for peripheral circuits. Therefore, it is actually difficult to realize the driving elements for peripheral circuits by using the a-Si TFT.
Meanwhile, a polycrystalline silicon (hereinafter, referred to as “poly-Si”) TFT having an active layer made from poly-Si has a relatively high mobility of about several tens to several hundreds cm2/Vs, which satisfies such an operation speed as required by the driving elements for peripheral circuits. Therefore, a poly-Si layer formed on a glass substrate enables realization of not only pixel switching elements but also driving elements for peripheral circuits. Further, the poly-Si layer formed on a glass substrate eliminates the necessity for a separate module process for forming a peripheral circuit, and enables driving elements for peripheral circuits to be formed simultaneously while the pixel area is formed, thereby reducing the cost for forming the driving elements for peripheral circuits.
Further, the high mobility of the poly-Si TFT enables the poly-Si TFT to have a volume smaller than that of the a-Si TFT. Further, the driving elements for peripheral circuits and the switching element in the pixel area can be simultaneously formed through an integration process, so that it becomes easier to reduce the line width and very advantageous to obtain a high resolution which it is otherwise difficult to obtain in the a-Si TFT-LCD.
Moreover, the high current characteristic of the poly-Si TFT enables the poly-Si TFT to be proper for a driving element of an organic light emitting display device, which is a flat panel display device of the next generation. Therefore, active researches are currently concentrated on the poly-Si TFT having a poly-Si layer formed on a glass substrate.
Here, according to one of the methods for forming a poly-Si layer on a glass substrate, a-Si layer is deposited on a glass substrate and is then subjected to heat treatment, so that the a-Si layer is crystallized. However, this method may cause deformation of the glass substrate at a temperature above 600° C., thereby degrading reliability and reducing yield of the products.
Therefore, an excimer laser annealing has been proposed as a method capable of crystallizing only the a-Si layer without causing thermal damage to the glass substrate. Also, a Sequential Lateral Solidification (hereinafter, referred to as “SLS”) method has been proposed.
In the SLS method, an a-Si layer and a poly-Si layer are crystallized by means of a pulse laser and a mask having a slit pattern for selectively providing transmission part, so that the layers can be crystallized in various types according to the shapes of the masks and the methods of progressing the crystallization.
Specifically, in the SLS method, a laser beam is shed through the transmission part of the mask, which includes not only the transmission part but also a non-transmission part, thereby partially melting the a-Si layer. Then, according to lapse of time, the molten part of the a-Si layer develops in a lateral direction while being converted into a poly-Si layer. Then, the above process is repeated while the substrate is moved, until the all of the a-Si layers are crystallized into the poly-Si layers. Since the SLS method can selectively crystallize only the a-Si layer without causing thermal damage to the glass substrate, the SLS method is very advantageous in the crystallization of the a-Si layer.
However, in the SLS method, while crystal grains develop, the crystal grains developing from the left side of the laser-shed area and the crystal grains developing from the right side thereof may collide with each other at a central portion of the laser-shed area, thereby forming a protrusion which stops the development of the crystal grains. Here, the protrusion has many defects because it is formed by the collision between two kinds of crystal grains having different orientations. Therefore, if a protrusion is formed in a channel area of a poly-Si TFT while the TFT is formed through crystallization of the a-Si layer using the SLS method, the protrusion largely degrades the mobility of electrons and holes when the TFT is operated. Also, if a protrusion is formed in a drain area of the poly-Si TFT, the current leakage increases. Therefore, even in a transistor having the same channel length and width, the TFT characteristic shows a large difference between the existence or absence of the protrusion, which becomes a source causing the TFT characteristic of the entire substrate to be non-uniform.